The long range objective of this work is to understand the mechanisms by which lentiviruses become resistant to antiviral drugs and to utilize this information to help develop improved strategies for chemotherapy of the acquired immune deficiency syndrome (AIDS). This information should be particularly important for development of approaches to combat the emergence of drug-resistance. We have developed model systems using feline immunodeficiency virus (FIV) for studies of resistance to AIDS chemotherapy. These systems have proven extremely useful and versatile for selection and characterization of FIV mutants resistant to nucleoside analogs. The work proposed here will extend these systems to address several important aspects of resistance: 1) Preliminary data suggest that factors other than reverse transcriptase (RT) are involved in resistance to 3'-azido-3'-deoxythymidine (AZT). The non-RT factors involved in AZT- resistance will be identified and characterized. 2) The effect of drug concentration on the types of drug-resistant mutants that arise will be determined. In particular, the emergence of mutants in the presence of subinhibitory concentrations of drugs will be studied. 3) The FIV model will be improved by introduction of the human immunodeficiency virus type 1 (HIV-1) RT into FIV. The resulting chimeric virus will enable studies of this important enzyme in our versatile model system. 4) An exciting new class of oligonucleotide (RNA) inhibitors that are targeted against RT will be studied. Experiments will be carried out to select and characterize mutants resistant to these inhibitors. Approaches will be initiated using these inhibitors to evaluate strategies that are designed to combat resistance.